1. Consisting of or resembling mother-of-pearl
2. Iridescent or opalescent.
Etymology: nacre is another word for mother-of-pearl, the iridescent inner layer of some mollusk shells. 'Nacre' appears to come from French.
When I was young, I would spend every summer day in the woods by the river, often collecting shells. A particularly nacreous shell was nice, but even better was to find two halves that fit together: I'd take those home, with plans of turning them into fancy boxes. Mom was very tolerant of me repeatedly surrounding the bathroom sink with freshly-scrubbed shells and rocks.